The monitoring of intracellular oxidative metabolism, pH and PO2 will be carried out at five levels: 1) excised rabbit papillary muscles, 2) isolated purkinje fibers, 3) ventricular strips of the toad, 4) the perfused rabbit septum, and 5) the intact dog heart in situ. The effects of hypoxia-anoxia will be studied in all these preparations and correlated with the excitability and mechanical properties of the tissue. In addition, in the last two preparations these effects will be compared with those of ischemia. BIBLIOGRAPHIC REFERENCES: "An Initial Study of the Effects of Pyrenebutyric Acid on Rabbit Heart Mitochondria," Snow, T.R. and F.F. Jobsis, Paper presented at the Federation of American Societies for Experimental Biology (Abstr. #1462) Atlantic City, NJ; April 1975. "Characterization of the Non-invasive Oxygen Probe Pyrenebutyric Acid in Heart Mitochondria," Snow, T.R. and F.F. Jobsis, Paper presented at the Fall Meeting of the American Physiological Society, San Franscisco, CA; October, 1975.